ELSEVIER Synthetic Metals103(1999) 2306-2307 Pyridinyl radicals as precursors of benzylic polyradicals IL Okada*, H. Mori, M. Kozaki, K. Sato, and T. Takui Department of Chemistry, Graduate School of Science, Osuku City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan Abstract A new photochemical method for the generation of benzylic high-spin specieswas developed using GN bond cleavage of pyridinyl radicals. The generation of m-xylylene and l,&naphthoquinodimethane were exemplified and the method was successfully applied to generatenew diradicals, 2,4-dimethylenethiophene and 2,4-dimethylenefuran. Keywords: Electoron spin resonance,UV-Vis-NIR absorption, Heterocyclic synthesis 1. Introduction Matrix photolysis with extrusion of small molecules has been known as a powerful tool for the generation of unstable high-spin species. This method requires the preparation of specially designed precursors such as azo- or d&o-compounds. Platz et al. have reported a convenient method using photo-electron transfer method in matrix. [l] However, the generality of the electron transfer method has not been well documented. In the course of our study on high-spin molecules, we have been interested in u- bond cleavage reactions at low temperature. The cleavage of one o-bond usually producestwo radical centers and the detection of the desired radical may be obstructed by the presence of the concomitantly produced radical partner. However, the homolysis of the o-bond l3to the radical center may produce single radical center and spin-paired species. Ikegami et al. reported such an interesting reaction of pyridinyl radical (eq 1). [2] This reaction may be extended to the generation of high-spin species. We here apply this reaction to the generation of benzylic high-spin species. C02Me hu 77 K in MTHF C02Me (1) 2. Results and Discussion 2.1 Demonstration of new photochemical procedure via C-N bond cleavage reaction of pyridinyl radical To demonstrate the applicability of the present method, two diradicals, m-xylylene (la) and 1,8-naphtoquinodimethane (lb) were chosen. These diradicals have been well characterizedby ESR spectroscopy and their zfs parameters are considerably different: ID/hcj = 0.011 cm-l, jE/Jzcl s 0.001 cm-1 for la [3], ]D/hcl = 0.011 cm-l, IE/Izcj 5 0.001 cm-l for lb [4]. The strategy of the generation of the diradicals la, lb was outlines in Scheme 1. The bispyridinium saits Za,b2+ were prepared in good yields (60-80%) by the reaction of a,a’- dibromo-m-xylene (3a) or %a’-dibromo-1,8-dimethylnaphthalene (3b) with 4-carboalkoxypyridines. The reduction of 2a,bZt. was achieved by shaking the a&o&rile solution with 3% Na-Hg alloy under the degassed conditions at room temperature. The progress of the reduction was monitored by measuring an electronic spectrum as a function of contact time. Fig. 1. illustrates the change of electronic spectrum in the reduction of 2aZi.. The colorless solution of the 2a2+ turned deep green and new absorptions (hmsx = 303 nm, 389 nm), which are quite similar to the known 4-carboalkoxy- pyridinyl monoradical absorption [2], increased the intensity as reduction proceeded. When the increase of the absorption was saturated, the reduction was stopped. The resulting specieswere assigned as the biradical 2a. The bispyridinyl diradical 2a was stable at room temperature. The similar procedure was used for the preparation of diradical 2b. The absorption maxima of these diradicalswere included in Table 1. Scheme 1 Q 2x n M-l& WC~N% H2C,N3C02R 3a,b 2a,b2* i reduction H&H A RO,C@CH, ‘H@~CO,R Q 2. .2 la,b 2a,b a: 5-i 0: benzene-l ,3-diyl, R: Me, b:@: naphthalene-I ,8-djyl, A: 3u ESR spectrum of 2a (in MTHF after replacing the solvent) before irradiation at 10K [Fig. 2. (a) for 2aj shows a strong central signal which has no splitting. Irradiation of 2a in ESR probe with a 500 W Xenon lamp (a400 run) gave new absorptions besides the central signal. 0379-67791991s - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00648-l